1. Field of the Invention
The invention relates generally to lightguides such as used for illuminating display devices, and more particularly, the invention relates to a lightguide having a directly secured reflector.
2. Description of the Related Technology
Backlit display devices, such as liquid crystal display (LCD) devices, commonly use a slab or wedge-shaped lightguide. The lightguide distributes light from a substantially linear source, such as a cold cathode fluorescent lamp (CCFL), to a substantially planar output. The planar light output of the lightguide is used to illuminate the LCD.
A measure of the performance of the display device is its brightness, and each of the components of the display device contributes, either positively or negatively, to its brightness. From a subjective standpoint, relatively small increases or decreases in overall brightness are not easily perceived by the end user of the display device. However, the display device designer is discouraged by even the smallest decreases in overall brightness including decreases so small they might only be perceived by objective measurement. This is because the display brightness and the power requirements of the display are closely related. If overall brightness can be increased without increasing the required power, the designer can actually allocated less power to the display device, yet still achieve an acceptable level of brightness. For battery powered portable devices, this translates to longer running times.
As is well known, the purpose of the lightguide is to provide for the distribution of light from the light source over an area much larger than the light source, and more particularly, substantially over an entire output surface area of the lightguide. In slab, wedge and pseudo-wedge lightguides, light typically enters the lightguide along an edge surface and propagates between a back surface and the output surface from the edge surface toward an opposing end surface of the lightguide by total internal reflection (TIR). In slab and certain wedge lightguides, the back surface includes structures, e.g., dots in a pattern, facets, etc. A light ray encountering one of these structures is redirected, i.e., either diffusely or specularly reflected, in such a manner that it is caused to exit the output surface. In other wedge lightguides, light is extracted by frustration of the TIR. A ray confined within the lightguide by TIR increases its angle of incidence relative to the plane of the top and bottom wall, due to the wedge angle, with each TIR bounce. The light eventually refracts out of the output surface at a glancing angle thereto, because it is no longer contained by TIR.
Some light rays also exit the back surface of the lightguide. Light rays also may be reflected by other elements of the display system back into the lightguide, and these light rays continue through the lightguide toward and through the back surface. To prevent the light rays that exit or pass through the back surface from being lost, it is known to provide a reflector adjacent the back surface to reflect these rays back into and through the lightguide. By providing the reflector the brightness of the lightguide is increased.
The light rays exiting the back surface of the wedge lightguide by frustration of TIR do so at an exit angle of about 65xc2x0-85xc2x0. Especially in the 80xc2x0-85xc2x0 range, the percentage of exiting light rays is low due to the constraints of Fresnel surface reflection near the critical angle. However, once these rays have exited it is equally difficult for them to reenter the lightguide when reflected between the back surface and the parallel positioned specular reflector. The result is the light becoming trapped between the lightguide back surface and the reflector. With every encounter of the light rays with the reflector, losses occur. The result is a loss of light that might otherwise be used to contribute to display brightness.
In one aspect of the invention a lightguide includes an input edge surface, a back surface and an output surface. A reflector is directly secured to the back surface by, for example, adhesive bonding.
In another aspect of the invention, an illumination device includes a lightguide having an integral back reflector.
In still another aspect of the invention, a reflector is secured to a back surface of a lightguide by a patterned layer of adhesive.
In yet another aspect of the invention, a polarizer is directly secured to a lightguide by a layer of adhesive.
A method of making a lightguide with a directly secured reflector is also encompassed by the invention.